Make Geary's UI responsive to screen size

Current status:

• Main window
• Composer
• Preferences
• Accounts

---

Adrien's original description:

Let's make Geary work on Mobile phones for the release of GNOME 3.32. :) Initiatives#5

Here is my preliminary version, including no navigation in the UI yet: https://gitlab.gnome.org/Community/Purism/geary/tree/wip/aplazas/adaptive